Project Summary Falling and injuries resulting from falls are pervasive among transfemoral amputees, who are particularly susceptible to such falls due to deficiencies in current prosthetic interventions. The purpose of this study is to: 1) better understand the factors that lead to falls in amputees, and 2) investigate the potential that active reflexive behaviors in a prosthesis will decrease the likelihood that stumble perturbations will result in a fall. Specifically, this study will: 1) experimentally characterize the likelihood that stumble perturbations will result in a fall in the transfemoral amputee population when using typical daily-use knee prostheses; 2) characterize the likelihood that stumble perturbations will result in a fall when the knee prosthesis is endowed with a stretch- reflex-type behavior; and 3) characterize the likelihood that stumble perturbations will result in a fall when the knee prosthesis is endowed with a stumble-recovery-reflex-type behavior. The first study will provide important data regarding the susceptibility of transfemoral amputees to stumble perturbations, and will enable the research community to focus attention on the regions of highest susceptibility (i.e., the portions of gait phase during which a stumble perturbation is most likely to result in a fall) in this population. Focusing on regions of highest susceptibility will presumably result in more targeted and effective interventions for reducing the likelihood that stumble perturbations result in falls. The second and third studies will investigate the potential of two such interventions to mitigate falls in this population, and will compare their efficacy to the baseline study. The results of the study described here could have a substantial impact in reducing transfemoral amputee falls, and as a result could substantially improve health and quality of life of this population.